So why did I say three browsers when there are four different rendering engines. The problem is that while you can test with all four on Windows (which supports Internet Explorer, Firefox, Opera, and Safari - as well as many other browsers) there is no way to test all four if you use a Mac or Linux.
Since the demise of Internet explorer on the Mac back in 2003 (which in any case did not use Trident) there has been no version of Internet Explorer that runs on any operating system except Windows. In any case Trident itself is so closely embedded into Windows itself that it cannot be used on any other operating system anyway. This means that say Firefox, Opera, and Safari on a Mac or Firefox, Opera, and Konqueror on Linux would be the three browsers you can test with and you have to rely on someone with access to Windows to test it in Internet Explorer for you.